The present invention relates to a golf ball which has numerous dimples on the surface and is endowed with a high symmetry performance.
Numerous dimples are usually arranged on the surface of a golf ball. These dimples are formed by using a two-part mold having a spherical cavity defined by an inside wall having numerous dimple-forming projections thereon, which spherical cavity splits into upper and lower hemispherical halves.
When numerous dimples are formed on the surface of the ball by a molding process such as injection molding using a two-part mold, it has been difficult to place dimples on the parting line of the mold. A belt-like land area called the seam line thus appears at the equator on the surface of the ball.
A number of innovations have been proposed recently for further enhancing the aerodynamic performance of the ball due to the dimples, and for tightly arranging the dimples even on the seam line.
In addition, efforts are being made to achieve substantially the same effective total dimple volume, regardless of whether the golf ball is “pole shot” so that it spins pole over pole (with the axis of rotation lying on the equatorial plane) or “seam shot” so that it spins pole horizontal (with the axis of rotation being the line connecting the poles) (see JP-B 6-7875).
However, because a golf ball is generally molded with a mold, at the interior of which is formed a spherical cavity, by separably mating together two equally split mold halves and is fabricated with axial symmetry, the ball tends to have a high degree of roundness about a polar axis corresponding to a line that connects the respective vertices within the cavities of the two mold halves, and conversely tends to have a low degree of roundness about an axis present on the plane which is circumscribed by the seam line and corresponds to the parting surface of the mold. In prior-art golf balls, owing to such differences in roundness, the flight performance of a ball will sometimes vary depending upon which part of the ball is struck by the club when the ball is played. In competitive play where, according to the rules of the game, a ball cannot be moved except under special circumstances and must be played in exactly the condition in which it is found, such variability in the flight performance is a major concern.
That is, when a golf ball is hit, although the spin rate of the ball differs depending on the number of the golf club, a back spin invariably arises. Referring to FIGS. 6A and B, depending on what part of the ball is hit, shots taken with the ball can be broadly divided into “pole shots” (FIG. 6A) in which the golf ball a is struck f in such a manner as to give rise to backspin about an axis of rotation which is a straight line e that connects three points: two mutually opposed points c and a on the seam line b of the ball a and a center point d; and “seam shots” (FIG. B) in which the golf ball a is struck i in such a way as to give rise to backspin about an axis of rotation which is a straight line h that is orthogonal to the circular flat plane g circumscribed by the seam line b of the ball a and passes through the center d of the ball a. As noted above, in a pole shot (FIG. 6A), the ball lacks roundness about the axis of rotation e, and thus is readily subject to superfluous lift and drag forces. On the other hand, in a seam shot (FIG. 6B), the ball is almost perfectly round about the axis of rotation h, and thus experiences substantially no superfluous lift and drag forces. Therefore, if a golf ball has simply been designed so that the dimple effects on pole shots and on seam shots are equivalent, disruption of the roundness on a pole shot will lead to a larger dimple effect, subjecting the golf ball to superfluous lift and drag and resulting in a flight performance which differs from that of a seam shot. Variations in the flight performance will thus arise depending on where the ball it hit.
Therefore, to obtain a golf ball having an excellent symmetry performance with no variation in flight performance depending on where the ball is hit, it has been desired that the dimple arrangement and dimple shapes be designed with the roundness of the ball in mind so as to optimize the dimple effects.
Various art that calibrates the symmetry performance has been disclosed, such calibration predominantly being carried out in the equatorial region of the ball. For example, JP-A 2000-325499 discloses art in which, by providing numerous dimples in the equatorial region of the ball which are deeper than the dimples present in other regions, dimples in the equatorial region are formed to an average depth which is from 5 to 50 μm greater than the average depth of dimples in the other regions. JP-A 10-99468 describes art in which dimples that lies across the parting line are given an average value for depth/diameter which is larger than the corresponding average value for dimples not on the parting line.
In the foregoing art, the symmetry performance is enhanced by making the dimples arranged on or near the ball equator deeper than dimples in other regions, but when the dimples near the equator are made too deep, the molded ball is more difficult to remove from the mold, resulting in a lower work efficiency. As a result, such art lacks practical effectiveness in golf ball production.